1. Field of the Invention
The present invention relates generally to a method and system for removing contaminants from solid substrates. More particularly, the present invention relates to such a method and system for use with an oxidant such as a hydrogen peroxide composition for removing contaminants such as solder flux and ionic species.
2. Description of the Background Art
Solder is used in the assembly of printed circuit boards, integrated circuits, and various electronic components and devices, in order to join metal parts together. The solder itself comprises an alloy, such as of indium or tin and lead. Before application of the solder to the metal surface, the surface is treated with a fluxing agent to remove oxides and other contaminants which might interfere with the metal joining, and to prevent reformation thereof during the solder process. These fluxing agents are typically organic materials, such as natural rosin extracted from pine tree sap, organic acids such as carboxylic acid, amines and amides, or inorganic materials, such as inorganic acids or salts. The most commonly used fluxing agent is acid rosin flux. The term "rosin flux" is used herein to mean a flux material which comprises rosin, i.e., the resin after distilling turpentine from the exudation of species of pine, and containing abietic acid and its anhydride. Typically, a small amount of another acid is added to rosin flux to activate it and such compositions are referred to as "acid rosin flux" or "rosin activated flux." After the solder operation has been completed, excess flux as well as flux residues remain on the soldered surface, and these residues are resinous, waxy, corrosive and conductive. These flux residues and excess flux must be removed prior to subsequent processing steps in order to prevent reaction thereof with the soldered parts, leading to corrosion and resultant electrical insulation losses. In particular, any ionic species, such as those derived from adipic acid or halogens, are electrically conductive and would degrade electronic circuit performance if not removed. These ionic species are carried by the organic flux residue.
Defluxing agents which are currently widely used are halogenated hydrocarbons such as 1,1,1-trichloroethane and Freon (a tradename of E. I. DuPont de Nemours for polyhalogenated hydrocarbons including chlorine and fluorine). While these organic materials are effective defluxing solvents, they have the serious disadvantage that they have a negative impact on the environment due to ozone depletion. Recent environmental legislation mandates that the production of these materials be severely restricted in the next few years and eventually eliminated. When these materials are used, even in small quantities, expensive management systems for transport, storage, use, and disposal and environmental protection equipment must be used to prevent air and water discharges. In addition, waste solvents require energy-intensive regeneration operations for these materials.
Numerous aqueous cleaners are available which are viable defluxing solvents; however, none of these has been found to be meet the stringent cleaning and compatibility requirements of a high reliability electronic assembly defluxing solvent. This is particularly relevant to assemblies with close tolerance spaces. In addition, the resulting organic-laden aqueous solvents require further processing before disposal.
Thus, an urgent need exists in the electronics industry for a solvent which effectively removes flux residues and contaminants while at the same time avoiding any negative environmental impact. Such a solvent would also be useful for removing other contaminants from other substrates. These requirements are met by the composition and method described in copending patent application Ser. No. 07/700,353, filed May 7, 1991, and copending patent application Ser. No. 07/583,911, filed Sep. 27, 1990, both of which are assigned to the present assignee. In accordance with the invention described in the latter two patent applications, there is first provided a composition comprising: (a) hydrogen peroxide in the amount of about 3 to 5 percent by weight of said composition; (b) an alkali metal hydroxide in sufficient amount to provide a pH of at least 10.5 in said composition; (c) a chosen wetting agent in the amount of about 0.1 to 0.3 percent by weight of said composition, said wetting agent being unreactive with said hydrogen peroxide and said alkali metal hydroxide; and (d) purified water as the balance of said composition. The solid substrate having organic contaminants thereon is exposed to the above-noted composition whereby the organic contaminants are removed from the substrate and are converted into non-toxic and non-hazardous products. Thus, negative environmental impact is avoided by this process. While this composition and method work well for their intended purpose, it would be desirable to provide still further modifications and improvements to them for greater efficiency and reduced cost.